Active air-fuel ratio control is performed to alternately control an air-fuel ratio in an area located upstream of a catalyst between a lean side and a rich side. The amount of oxygen absorbed and released by the catalyst during lean control and rich control of the air-fuel ratio is measured. When an output from the post-catalyst sensor reaches a predetermined threshold, switching is carried out between the lean control and the rich control, and the measurement of the amount of oxygen is ended. The threshold is set to be reached before the output from the post-catalyst sensor reaches a predetermined reference value. Whether the catalyst is normal or abnormal is determined based on the amount of oxygen measured until the output from the post-catalyst sensor reaches the threshold and behavior of the output from the post-catalyst sensor observed after the output from the post-catalyst sensor reaches the threshold.